The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Conventionally, cylindrical spur and helical gear pairs are routinely utilized to transfer torque and speed between parallel shafts. Bevel gear pairs are routinely utilized to transmit torque between a pair of shafts having intersecting axes that are disposed at an angle to one another, commonly at a right angle. Likewise, so called “on-center” face gear pairs can be use to transfer torque between intersecting shafts. Such a gear arrangement consists of a spur cylindrical pinion and a face gear mounted like bevel gears on shafts that intersect orthogonally to each other. When the cylindrical pinion teeth mesh with face gear teeth they act as bevel gears, their pitch surfaces being cones of rotation. Apart from the fact that they are less sensitive at mounting distance than bevel gears are, their main advantage is that the pinion bearings carry mostly radial load, while the gear bearings have both radial and thrust load. Due to operating pressure increases towards the outside diameter variation, while the depth of the tooth remains constant, the maximum usable outside diameter is the diameter at which the teeth become pointed. At the inside end, the limit is the radius at which the undercut becomes excessive.
It is generally a more difficult challenge to transmit torque between skew-axes shafts, that are neither parallel nor intersecting, and more particularly to non-intersecting shafts disposed “offset” at right angles. A significant difficulty is to transmit the torque and speed between non-intersecting orthogonal axes in both directions of rotation of the driving member, and moreover, to be able to interchange the two components status from drive to driven and vice-versa, within the same gear drive system. While most of skew-axis drives are “bi-rotational” not many can be “bi-directional” at the same time. As utilized herein, the term “bi-rotational” means that the gear assembly can transmit torque in either direction of rotation of the driving member, while the term “bi-directional” refers to whether a specific gear, e.g., the pinion gear, can operate as either the input or drive gear or the output or driven gear as well. Sometimes, within gear dedicated technical literature, the ability of a gear drive system to be “non-bidirectional” is called “self-locking” or “anti-backdrive.”
The most common prior art approach to the problem of torque transfer between non-intersecting orthogonal shafts involves the use of hypoid gears, especially in typical automobile differentials. They resemble bevel gears in some respects, but differ from true bevel gears in that their axes do not intersect. The distance between a hypoid pinion (in all practical cases, the driving member) axis and the axis of a hypoid gear (in all practical cases, the driven member) is called “offset”. Hypoid pinions may have as few as five teeth, compared with bevel gears that do not often have fewer than ten teeth, a fact that suggests their ability to realize high reduction gear ratios, and that they are seldom used as gear ratio multipliers. Hypoid gears are especially suited for transmitting large amounts of torque through angles with good efficiency and improved contact load capacity as disclosed by U.S. Pat. No. 2,961,888. Their use has also drawbacks. Hypoid gears are extremely sensitive to their components' relative location, being also adversely affected by small amounts of thermal growth during operation, as well as by defections of the gear supporting structure under load. Such sensitivity requires also complex manufacturing, assembly, and gear mating procedures, limiting the speed reduction ratios for which they can be advantageously employed to low gear speed reduction ratios (ratios less than 4:1).
Hereinafter, the term “pinion” will be used for a gear drive system component, having the teeth disposed radially on a cylindrical or conical surface that has the axis of rotation identical with axis of rotation of the corresponding component. Usually, it is the smaller component in mesh and can be a cylindrical gear with helical teeth, a cylindrical gear with curved teeth along its axis, or a cylindrical or conical worm. The term “gear”, “face gear” or “crown gear” is usually used for the larger component in mesh having the axis of rotation disposed offset at a right angle relative to the pinion axis, and the teeth formed on one of its side faces. The teeth can be disposed non-radially, having straight flanks or curved along a spiral curve, thus being provided with a convex and a concave flank. The tooth top land can lie in a plane, and the gear hence may be called a “flat gear” or on the surface of a cone. Likewise, the bottom land of gear tooth can lie also in a plane or on a surface of a cone.
A common prior art approach to the problem of torque transfer between non-intersecting orthogonal shafts involves the use of offset face gears, as partially disclosed by U.S. Pat. No. 5,178,028. The teeth of a cylindrical pinion disposed offset at a right angle relative to the face gear axis are helical, while the face gear teeth having straight edges are formed on one of its end faces and inclined from radial direction. The top and bottom land lie in two parallel planes. Although this type of gear system exhibits less sensitivity to the axial position of the pinion on the face gear, as well as more tolerance for movement toward or away from its driven face gear compared to an equivalent hypoid gear pair, due to the offset, the profile variations of face gear teeth are more pronounced. Like on-center face gears, the offset face gears have their outside and inside diameters limited by teeth pointing and undercutting phenomena. They can easily accommodate speed reduction ratios greater than 4:1. In another embodiment, as disclosed by U.S. Pat. No. 2,311,006, a spiral crown gear having a plurality of longitudinally curved teeth of ever increasing radii of curvature and a variable cross section throughout their length meshes with a cylindrical pinion having a plurality of helical teeth of a constant cross section throughout their length, though its pitch surface is a hyperboloid. Due to teeth asymmetry of both components, the novel gear drive allows for interference avoidance and for a prolonged contact. There is no mention of their bi-directional capability.
Another common prior art approach to the problem of torque transfer between non-intersecting orthogonal shafts involves the use of skew-axis gearings of so-called “worm-face gears drives” type. Characteristic of this type of gears is their high gear ratio in a compact arrangement and their good load-carrying capacity. Often they are recognized by their trademark names, the most known being Spiroid®, Helicon®, and Spiradrive® gear systems. Specific for all gear drives where the axes are either parallel or intersecting, the pitch diameters of the mating gears must be exactly proportional to their respective number of teeth and inversely proportional to the relative velocities. In this case of offset gear drives the respective pitch diameters are independent of the gear ratio. Where one of the members is a worm, as disclosed in U.S. Pat. No. 1,683,758, its pitch diameter may be changed at will by altering its thread angle. Such gears thus have the advantage that the driving member or the worm may be made proportionately larger compared to bevel gears with intersecting axes having the same gear ratio. Here the crown gear having longitudinally curved teeth of constant height mates with a cylindrical worm, the worm threads and the gear teeth being of opposite hand.
U.S. Pat. No. Re. 16,137 discloses a conventional gear system in which a conical worm or a beveled pinion meshes with a spiral beveled gear. The beveled gear teeth are shaped in the form of modified involutes of a circle. Specific for both mentioned gear systems, is the fact that the worm is the primary member. U.S. Pat. No. 2,896,467 discloses another conventional gear system capable of an unusually large offset, great area of contact and low reduction ratios. In this case, the gear is considered the primary member, rather than the worm. While the worm threads are curved, the face gear teeth are straight and non-radially disposed on one end face of the gear, with the top land in a plane and the bottom lands on a conical surface. A significant difficulty of all these combinations, although it can sometimes be a benefit, is that torque transfer can occur only from the worm to the beveled gear—the worm gear cannot be back driven. Thus, such a gear assembly, although bi-rotational, is not bi-directional.
Gear configurations for non-intersecting orthogonal shafts that are both bi-rotational and bi-directional do exist. Prior art is disclosed by U.S. Pat. No. 4,238,970 where a bevolute gear system is designed to be completely non-self-locking. The gear system includes a non-beveled pinion having the teeth shaped in the form of an involute spiral, and meshes with spiral involute curved teeth of a face gear, positioned at approximately 90° relative to each other at an offset in the range of 50% to 75% of the pitch circle radius of the face gear. The pinion includes teeth which are shaped in the form of a normal involute spiral. The bevolute gear system includes a second gear which also includes teeth which are shaped in the form of a normal involute spiral and flat and in one plane, mounted on a non-intersecting axis at a right angle to the axis of the pinion gear. However, these configurations are typically limited as gears speed ratio span, means to avoid their teeth interference in mesh and undercutting in the manufacturing process, means to improve the gears load capacity and efficiency while reducing their weight, contact stress and noise in operation.